alloc/
boxed.rs

1//! The `Box<T>` type for heap allocation.
2//!
3//! [`Box<T>`], casually referred to as a 'box', provides the simplest form of
4//! heap allocation in Rust. Boxes provide ownership for this allocation, and
5//! drop their contents when they go out of scope. Boxes also ensure that they
6//! never allocate more than `isize::MAX` bytes.
7//!
8//! # Examples
9//!
10//! Move a value from the stack to the heap by creating a [`Box`]:
11//!
12//! ```
13//! let val: u8 = 5;
14//! let boxed: Box<u8> = Box::new(val);
15//! ```
16//!
17//! Move a value from a [`Box`] back to the stack by [dereferencing]:
18//!
19//! ```
20//! let boxed: Box<u8> = Box::new(5);
21//! let val: u8 = *boxed;
22//! ```
23//!
24//! Creating a recursive data structure:
25//!
26//! ```
27//! # #[allow(dead_code)]
28//! #[derive(Debug)]
29//! enum List<T> {
30//!     Cons(T, Box<List<T>>),
31//!     Nil,
32//! }
33//!
34//! let list: List<i32> = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));
35//! println!("{list:?}");
36//! ```
37//!
38//! This will print `Cons(1, Cons(2, Nil))`.
39//!
40//! Recursive structures must be boxed, because if the definition of `Cons`
41//! looked like this:
42//!
43//! ```compile_fail,E0072
44//! # enum List<T> {
45//! Cons(T, List<T>),
46//! # }
47//! ```
48//!
49//! It wouldn't work. This is because the size of a `List` depends on how many
50//! elements are in the list, and so we don't know how much memory to allocate
51//! for a `Cons`. By introducing a [`Box<T>`], which has a defined size, we know how
52//! big `Cons` needs to be.
53//!
54//! # Memory layout
55//!
56//! For non-zero-sized values, a [`Box`] will use the [`Global`] allocator for its allocation. It is
57//! valid to convert both ways between a [`Box`] and a raw pointer allocated with the [`Global`]
58//! allocator, given that the [`Layout`] used with the allocator is correct for the type and the raw
59//! pointer points to a valid value of the right type. More precisely, a `value: *mut T` that has
60//! been allocated with the [`Global`] allocator with `Layout::for_value(&*value)` may be converted
61//! into a box using [`Box::<T>::from_raw(value)`]. Conversely, the memory backing a `value: *mut T`
62//! obtained from [`Box::<T>::into_raw`] may be deallocated using the [`Global`] allocator with
63//! [`Layout::for_value(&*value)`].
64//!
65//! For zero-sized values, the `Box` pointer has to be non-null and sufficiently aligned. The
66//! recommended way to build a Box to a ZST if `Box::new` cannot be used is to use
67//! [`ptr::NonNull::dangling`].
68//!
69//! On top of these basic layout requirements, a `Box<T>` must point to a valid value of `T`.
70//!
71//! So long as `T: Sized`, a `Box<T>` is guaranteed to be represented
72//! as a single pointer and is also ABI-compatible with C pointers
73//! (i.e. the C type `T*`). This means that if you have extern "C"
74//! Rust functions that will be called from C, you can define those
75//! Rust functions using `Box<T>` types, and use `T*` as corresponding
76//! type on the C side. As an example, consider this C header which
77//! declares functions that create and destroy some kind of `Foo`
78//! value:
79//!
80//! ```c
81//! /* C header */
82//!
83//! /* Returns ownership to the caller */
84//! struct Foo* foo_new(void);
85//!
86//! /* Takes ownership from the caller; no-op when invoked with null */
87//! void foo_delete(struct Foo*);
88//! ```
89//!
90//! These two functions might be implemented in Rust as follows. Here, the
91//! `struct Foo*` type from C is translated to `Box<Foo>`, which captures
92//! the ownership constraints. Note also that the nullable argument to
93//! `foo_delete` is represented in Rust as `Option<Box<Foo>>`, since `Box<Foo>`
94//! cannot be null.
95//!
96//! ```
97//! #[repr(C)]
98//! pub struct Foo;
99//!
100//! #[unsafe(no_mangle)]
101//! pub extern "C" fn foo_new() -> Box<Foo> {
102//!     Box::new(Foo)
103//! }
104//!
105//! #[unsafe(no_mangle)]
106//! pub extern "C" fn foo_delete(_: Option<Box<Foo>>) {}
107//! ```
108//!
109//! Even though `Box<T>` has the same representation and C ABI as a C pointer,
110//! this does not mean that you can convert an arbitrary `T*` into a `Box<T>`
111//! and expect things to work. `Box<T>` values will always be fully aligned,
112//! non-null pointers. Moreover, the destructor for `Box<T>` will attempt to
113//! free the value with the global allocator. In general, the best practice
114//! is to only use `Box<T>` for pointers that originated from the global
115//! allocator.
116//!
117//! **Important.** At least at present, you should avoid using
118//! `Box<T>` types for functions that are defined in C but invoked
119//! from Rust. In those cases, you should directly mirror the C types
120//! as closely as possible. Using types like `Box<T>` where the C
121//! definition is just using `T*` can lead to undefined behavior, as
122//! described in [rust-lang/unsafe-code-guidelines#198][ucg#198].
123//!
124//! # Considerations for unsafe code
125//!
126//! **Warning: This section is not normative and is subject to change, possibly
127//! being relaxed in the future! It is a simplified summary of the rules
128//! currently implemented in the compiler.**
129//!
130//! The aliasing rules for `Box<T>` are the same as for `&mut T`. `Box<T>`
131//! asserts uniqueness over its content. Using raw pointers derived from a box
132//! after that box has been mutated through, moved or borrowed as `&mut T`
133//! is not allowed. For more guidance on working with box from unsafe code, see
134//! [rust-lang/unsafe-code-guidelines#326][ucg#326].
135//!
136//! # Editions
137//!
138//! A special case exists for the implementation of `IntoIterator` for arrays on the Rust 2021
139//! edition, as documented [here][array]. Unfortunately, it was later found that a similar
140//! workaround should be added for boxed slices, and this was applied in the 2024 edition.
141//!
142//! Specifically, `IntoIterator` is implemented for `Box<[T]>` on all editions, but specific calls
143//! to `into_iter()` for boxed slices will defer to the slice implementation on editions before
144//! 2024:
145//!
146//! ```rust,edition2021
147//! // Rust 2015, 2018, and 2021:
148//!
149//! # #![allow(boxed_slice_into_iter)] // override our `deny(warnings)`
150//! let boxed_slice: Box<[i32]> = vec![0; 3].into_boxed_slice();
151//!
152//! // This creates a slice iterator, producing references to each value.
153//! for item in boxed_slice.into_iter().enumerate() {
154//!     let (i, x): (usize, &i32) = item;
155//!     println!("boxed_slice[{i}] = {x}");
156//! }
157//!
158//! // The `boxed_slice_into_iter` lint suggests this change for future compatibility:
159//! for item in boxed_slice.iter().enumerate() {
160//!     let (i, x): (usize, &i32) = item;
161//!     println!("boxed_slice[{i}] = {x}");
162//! }
163//!
164//! // You can explicitly iterate a boxed slice by value using `IntoIterator::into_iter`
165//! for item in IntoIterator::into_iter(boxed_slice).enumerate() {
166//!     let (i, x): (usize, i32) = item;
167//!     println!("boxed_slice[{i}] = {x}");
168//! }
169//! ```
170//!
171//! Similar to the array implementation, this may be modified in the future to remove this override,
172//! and it's best to avoid relying on this edition-dependent behavior if you wish to preserve
173//! compatibility with future versions of the compiler.
174//!
175//! [ucg#198]: https://github.com/rust-lang/unsafe-code-guidelines/issues/198
176//! [ucg#326]: https://github.com/rust-lang/unsafe-code-guidelines/issues/326
177//! [dereferencing]: core::ops::Deref
178//! [`Box::<T>::from_raw(value)`]: Box::from_raw
179//! [`Global`]: crate::alloc::Global
180//! [`Layout`]: crate::alloc::Layout
181//! [`Layout::for_value(&*value)`]: crate::alloc::Layout::for_value
182//! [valid]: ptr#safety
183
184#![stable(feature = "rust1", since = "1.0.0")]
185
186use core::borrow::{Borrow, BorrowMut};
187#[cfg(not(no_global_oom_handling))]
188use core::clone::CloneToUninit;
189use core::cmp::Ordering;
190use core::error::{self, Error};
191use core::fmt;
192use core::future::Future;
193use core::hash::{Hash, Hasher};
194use core::marker::{PointerLike, Tuple, Unsize};
195use core::mem::{self, SizedTypeProperties};
196use core::ops::{
197    AsyncFn, AsyncFnMut, AsyncFnOnce, CoerceUnsized, Coroutine, CoroutineState, Deref, DerefMut,
198    DerefPure, DispatchFromDyn, LegacyReceiver,
199};
200use core::pin::{Pin, PinCoerceUnsized};
201use core::ptr::{self, NonNull, Unique};
202use core::task::{Context, Poll};
203
204#[cfg(not(no_global_oom_handling))]
205use crate::alloc::handle_alloc_error;
206use crate::alloc::{AllocError, Allocator, Global, Layout};
207use crate::raw_vec::RawVec;
208#[cfg(not(no_global_oom_handling))]
209use crate::str::from_boxed_utf8_unchecked;
210
211/// Conversion related impls for `Box<_>` (`From`, `downcast`, etc)
212mod convert;
213/// Iterator related impls for `Box<_>`.
214mod iter;
215/// [`ThinBox`] implementation.
216mod thin;
217
218#[unstable(feature = "thin_box", issue = "92791")]
219pub use thin::ThinBox;
220
221/// A pointer type that uniquely owns a heap allocation of type `T`.
222///
223/// See the [module-level documentation](../../std/boxed/index.html) for more.
224#[lang = "owned_box"]
225#[fundamental]
226#[stable(feature = "rust1", since = "1.0.0")]
227#[rustc_insignificant_dtor]
228#[doc(search_unbox)]
229// The declaration of the `Box` struct must be kept in sync with the
230// compiler or ICEs will happen.
231pub struct Box<
232    T: ?Sized,
233    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
234>(Unique<T>, A);
235
236/// Constructs a `Box<T>` by calling the `exchange_malloc` lang item and moving the argument into
237/// the newly allocated memory. This is an intrinsic to avoid unnecessary copies.
238///
239/// This is the surface syntax for `box <expr>` expressions.
240#[cfg(not(bootstrap))]
241#[rustc_intrinsic]
242#[rustc_intrinsic_must_be_overridden]
243#[unstable(feature = "liballoc_internals", issue = "none")]
244pub fn box_new<T>(_x: T) -> Box<T> {
245    unreachable!()
246}
247
248/// Transition function for the next bootstrap bump.
249#[cfg(bootstrap)]
250#[unstable(feature = "liballoc_internals", issue = "none")]
251#[inline(always)]
252pub fn box_new<T>(x: T) -> Box<T> {
253    #[rustc_box]
254    Box::new(x)
255}
256
257impl<T> Box<T> {
258    /// Allocates memory on the heap and then places `x` into it.
259    ///
260    /// This doesn't actually allocate if `T` is zero-sized.
261    ///
262    /// # Examples
263    ///
264    /// ```
265    /// let five = Box::new(5);
266    /// ```
267    #[cfg(not(no_global_oom_handling))]
268    #[inline(always)]
269    #[stable(feature = "rust1", since = "1.0.0")]
270    #[must_use]
271    #[rustc_diagnostic_item = "box_new"]
272    #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
273    pub fn new(x: T) -> Self {
274        return box_new(x);
275    }
276
277    /// Constructs a new box with uninitialized contents.
278    ///
279    /// # Examples
280    ///
281    /// ```
282    /// let mut five = Box::<u32>::new_uninit();
283    /// // Deferred initialization:
284    /// five.write(5);
285    /// let five = unsafe { five.assume_init() };
286    ///
287    /// assert_eq!(*five, 5)
288    /// ```
289    #[cfg(not(no_global_oom_handling))]
290    #[stable(feature = "new_uninit", since = "1.82.0")]
291    #[must_use]
292    #[inline]
293    pub fn new_uninit() -> Box<mem::MaybeUninit<T>> {
294        Self::new_uninit_in(Global)
295    }
296
297    /// Constructs a new `Box` with uninitialized contents, with the memory
298    /// being filled with `0` bytes.
299    ///
300    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
301    /// of this method.
302    ///
303    /// # Examples
304    ///
305    /// ```
306    /// #![feature(new_zeroed_alloc)]
307    ///
308    /// let zero = Box::<u32>::new_zeroed();
309    /// let zero = unsafe { zero.assume_init() };
310    ///
311    /// assert_eq!(*zero, 0)
312    /// ```
313    ///
314    /// [zeroed]: mem::MaybeUninit::zeroed
315    #[cfg(not(no_global_oom_handling))]
316    #[inline]
317    #[unstable(feature = "new_zeroed_alloc", issue = "129396")]
318    #[must_use]
319    pub fn new_zeroed() -> Box<mem::MaybeUninit<T>> {
320        Self::new_zeroed_in(Global)
321    }
322
323    /// Constructs a new `Pin<Box<T>>`. If `T` does not implement [`Unpin`], then
324    /// `x` will be pinned in memory and unable to be moved.
325    ///
326    /// Constructing and pinning of the `Box` can also be done in two steps: `Box::pin(x)`
327    /// does the same as <code>[Box::into_pin]\([Box::new]\(x))</code>. Consider using
328    /// [`into_pin`](Box::into_pin) if you already have a `Box<T>`, or if you want to
329    /// construct a (pinned) `Box` in a different way than with [`Box::new`].
330    #[cfg(not(no_global_oom_handling))]
331    #[stable(feature = "pin", since = "1.33.0")]
332    #[must_use]
333    #[inline(always)]
334    pub fn pin(x: T) -> Pin<Box<T>> {
335        Box::new(x).into()
336    }
337
338    /// Allocates memory on the heap then places `x` into it,
339    /// returning an error if the allocation fails
340    ///
341    /// This doesn't actually allocate if `T` is zero-sized.
342    ///
343    /// # Examples
344    ///
345    /// ```
346    /// #![feature(allocator_api)]
347    ///
348    /// let five = Box::try_new(5)?;
349    /// # Ok::<(), std::alloc::AllocError>(())
350    /// ```
351    #[unstable(feature = "allocator_api", issue = "32838")]
352    #[inline]
353    pub fn try_new(x: T) -> Result<Self, AllocError> {
354        Self::try_new_in(x, Global)
355    }
356
357    /// Constructs a new box with uninitialized contents on the heap,
358    /// returning an error if the allocation fails
359    ///
360    /// # Examples
361    ///
362    /// ```
363    /// #![feature(allocator_api)]
364    ///
365    /// let mut five = Box::<u32>::try_new_uninit()?;
366    /// // Deferred initialization:
367    /// five.write(5);
368    /// let five = unsafe { five.assume_init() };
369    ///
370    /// assert_eq!(*five, 5);
371    /// # Ok::<(), std::alloc::AllocError>(())
372    /// ```
373    #[unstable(feature = "allocator_api", issue = "32838")]
374    // #[unstable(feature = "new_uninit", issue = "63291")]
375    #[inline]
376    pub fn try_new_uninit() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
377        Box::try_new_uninit_in(Global)
378    }
379
380    /// Constructs a new `Box` with uninitialized contents, with the memory
381    /// being filled with `0` bytes on the heap
382    ///
383    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
384    /// of this method.
385    ///
386    /// # Examples
387    ///
388    /// ```
389    /// #![feature(allocator_api)]
390    ///
391    /// let zero = Box::<u32>::try_new_zeroed()?;
392    /// let zero = unsafe { zero.assume_init() };
393    ///
394    /// assert_eq!(*zero, 0);
395    /// # Ok::<(), std::alloc::AllocError>(())
396    /// ```
397    ///
398    /// [zeroed]: mem::MaybeUninit::zeroed
399    #[unstable(feature = "allocator_api", issue = "32838")]
400    // #[unstable(feature = "new_uninit", issue = "63291")]
401    #[inline]
402    pub fn try_new_zeroed() -> Result<Box<mem::MaybeUninit<T>>, AllocError> {
403        Box::try_new_zeroed_in(Global)
404    }
405}
406
407impl<T, A: Allocator> Box<T, A> {
408    /// Allocates memory in the given allocator then places `x` into it.
409    ///
410    /// This doesn't actually allocate if `T` is zero-sized.
411    ///
412    /// # Examples
413    ///
414    /// ```
415    /// #![feature(allocator_api)]
416    ///
417    /// use std::alloc::System;
418    ///
419    /// let five = Box::new_in(5, System);
420    /// ```
421    #[cfg(not(no_global_oom_handling))]
422    #[unstable(feature = "allocator_api", issue = "32838")]
423    #[must_use]
424    #[inline]
425    pub fn new_in(x: T, alloc: A) -> Self
426    where
427        A: Allocator,
428    {
429        let mut boxed = Self::new_uninit_in(alloc);
430        boxed.write(x);
431        unsafe { boxed.assume_init() }
432    }
433
434    /// Allocates memory in the given allocator then places `x` into it,
435    /// returning an error if the allocation fails
436    ///
437    /// This doesn't actually allocate if `T` is zero-sized.
438    ///
439    /// # Examples
440    ///
441    /// ```
442    /// #![feature(allocator_api)]
443    ///
444    /// use std::alloc::System;
445    ///
446    /// let five = Box::try_new_in(5, System)?;
447    /// # Ok::<(), std::alloc::AllocError>(())
448    /// ```
449    #[unstable(feature = "allocator_api", issue = "32838")]
450    #[inline]
451    pub fn try_new_in(x: T, alloc: A) -> Result<Self, AllocError>
452    where
453        A: Allocator,
454    {
455        let mut boxed = Self::try_new_uninit_in(alloc)?;
456        boxed.write(x);
457        unsafe { Ok(boxed.assume_init()) }
458    }
459
460    /// Constructs a new box with uninitialized contents in the provided allocator.
461    ///
462    /// # Examples
463    ///
464    /// ```
465    /// #![feature(allocator_api)]
466    ///
467    /// use std::alloc::System;
468    ///
469    /// let mut five = Box::<u32, _>::new_uninit_in(System);
470    /// // Deferred initialization:
471    /// five.write(5);
472    /// let five = unsafe { five.assume_init() };
473    ///
474    /// assert_eq!(*five, 5)
475    /// ```
476    #[unstable(feature = "allocator_api", issue = "32838")]
477    #[cfg(not(no_global_oom_handling))]
478    #[must_use]
479    // #[unstable(feature = "new_uninit", issue = "63291")]
480    pub fn new_uninit_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
481    where
482        A: Allocator,
483    {
484        let layout = Layout::new::<mem::MaybeUninit<T>>();
485        // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
486        // That would make code size bigger.
487        match Box::try_new_uninit_in(alloc) {
488            Ok(m) => m,
489            Err(_) => handle_alloc_error(layout),
490        }
491    }
492
493    /// Constructs a new box with uninitialized contents in the provided allocator,
494    /// returning an error if the allocation fails
495    ///
496    /// # Examples
497    ///
498    /// ```
499    /// #![feature(allocator_api)]
500    ///
501    /// use std::alloc::System;
502    ///
503    /// let mut five = Box::<u32, _>::try_new_uninit_in(System)?;
504    /// // Deferred initialization:
505    /// five.write(5);
506    /// let five = unsafe { five.assume_init() };
507    ///
508    /// assert_eq!(*five, 5);
509    /// # Ok::<(), std::alloc::AllocError>(())
510    /// ```
511    #[unstable(feature = "allocator_api", issue = "32838")]
512    // #[unstable(feature = "new_uninit", issue = "63291")]
513    pub fn try_new_uninit_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
514    where
515        A: Allocator,
516    {
517        let ptr = if T::IS_ZST {
518            NonNull::dangling()
519        } else {
520            let layout = Layout::new::<mem::MaybeUninit<T>>();
521            alloc.allocate(layout)?.cast()
522        };
523        unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
524    }
525
526    /// Constructs a new `Box` with uninitialized contents, with the memory
527    /// being filled with `0` bytes in the provided allocator.
528    ///
529    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
530    /// of this method.
531    ///
532    /// # Examples
533    ///
534    /// ```
535    /// #![feature(allocator_api)]
536    ///
537    /// use std::alloc::System;
538    ///
539    /// let zero = Box::<u32, _>::new_zeroed_in(System);
540    /// let zero = unsafe { zero.assume_init() };
541    ///
542    /// assert_eq!(*zero, 0)
543    /// ```
544    ///
545    /// [zeroed]: mem::MaybeUninit::zeroed
546    #[unstable(feature = "allocator_api", issue = "32838")]
547    #[cfg(not(no_global_oom_handling))]
548    // #[unstable(feature = "new_uninit", issue = "63291")]
549    #[must_use]
550    pub fn new_zeroed_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
551    where
552        A: Allocator,
553    {
554        let layout = Layout::new::<mem::MaybeUninit<T>>();
555        // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
556        // That would make code size bigger.
557        match Box::try_new_zeroed_in(alloc) {
558            Ok(m) => m,
559            Err(_) => handle_alloc_error(layout),
560        }
561    }
562
563    /// Constructs a new `Box` with uninitialized contents, with the memory
564    /// being filled with `0` bytes in the provided allocator,
565    /// returning an error if the allocation fails,
566    ///
567    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
568    /// of this method.
569    ///
570    /// # Examples
571    ///
572    /// ```
573    /// #![feature(allocator_api)]
574    ///
575    /// use std::alloc::System;
576    ///
577    /// let zero = Box::<u32, _>::try_new_zeroed_in(System)?;
578    /// let zero = unsafe { zero.assume_init() };
579    ///
580    /// assert_eq!(*zero, 0);
581    /// # Ok::<(), std::alloc::AllocError>(())
582    /// ```
583    ///
584    /// [zeroed]: mem::MaybeUninit::zeroed
585    #[unstable(feature = "allocator_api", issue = "32838")]
586    // #[unstable(feature = "new_uninit", issue = "63291")]
587    pub fn try_new_zeroed_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
588    where
589        A: Allocator,
590    {
591        let ptr = if T::IS_ZST {
592            NonNull::dangling()
593        } else {
594            let layout = Layout::new::<mem::MaybeUninit<T>>();
595            alloc.allocate_zeroed(layout)?.cast()
596        };
597        unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
598    }
599
600    /// Constructs a new `Pin<Box<T, A>>`. If `T` does not implement [`Unpin`], then
601    /// `x` will be pinned in memory and unable to be moved.
602    ///
603    /// Constructing and pinning of the `Box` can also be done in two steps: `Box::pin_in(x, alloc)`
604    /// does the same as <code>[Box::into_pin]\([Box::new_in]\(x, alloc))</code>. Consider using
605    /// [`into_pin`](Box::into_pin) if you already have a `Box<T, A>`, or if you want to
606    /// construct a (pinned) `Box` in a different way than with [`Box::new_in`].
607    #[cfg(not(no_global_oom_handling))]
608    #[unstable(feature = "allocator_api", issue = "32838")]
609    #[must_use]
610    #[inline(always)]
611    pub fn pin_in(x: T, alloc: A) -> Pin<Self>
612    where
613        A: 'static + Allocator,
614    {
615        Self::into_pin(Self::new_in(x, alloc))
616    }
617
618    /// Converts a `Box<T>` into a `Box<[T]>`
619    ///
620    /// This conversion does not allocate on the heap and happens in place.
621    #[unstable(feature = "box_into_boxed_slice", issue = "71582")]
622    pub fn into_boxed_slice(boxed: Self) -> Box<[T], A> {
623        let (raw, alloc) = Box::into_raw_with_allocator(boxed);
624        unsafe { Box::from_raw_in(raw as *mut [T; 1], alloc) }
625    }
626
627    /// Consumes the `Box`, returning the wrapped value.
628    ///
629    /// # Examples
630    ///
631    /// ```
632    /// #![feature(box_into_inner)]
633    ///
634    /// let c = Box::new(5);
635    ///
636    /// assert_eq!(Box::into_inner(c), 5);
637    /// ```
638    #[unstable(feature = "box_into_inner", issue = "80437")]
639    #[inline]
640    pub fn into_inner(boxed: Self) -> T {
641        *boxed
642    }
643}
644
645impl<T> Box<[T]> {
646    /// Constructs a new boxed slice with uninitialized contents.
647    ///
648    /// # Examples
649    ///
650    /// ```
651    /// let mut values = Box::<[u32]>::new_uninit_slice(3);
652    /// // Deferred initialization:
653    /// values[0].write(1);
654    /// values[1].write(2);
655    /// values[2].write(3);
656    /// let values = unsafe {values.assume_init() };
657    ///
658    /// assert_eq!(*values, [1, 2, 3])
659    /// ```
660    #[cfg(not(no_global_oom_handling))]
661    #[stable(feature = "new_uninit", since = "1.82.0")]
662    #[must_use]
663    pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
664        unsafe { RawVec::with_capacity(len).into_box(len) }
665    }
666
667    /// Constructs a new boxed slice with uninitialized contents, with the memory
668    /// being filled with `0` bytes.
669    ///
670    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
671    /// of this method.
672    ///
673    /// # Examples
674    ///
675    /// ```
676    /// #![feature(new_zeroed_alloc)]
677    ///
678    /// let values = Box::<[u32]>::new_zeroed_slice(3);
679    /// let values = unsafe { values.assume_init() };
680    ///
681    /// assert_eq!(*values, [0, 0, 0])
682    /// ```
683    ///
684    /// [zeroed]: mem::MaybeUninit::zeroed
685    #[cfg(not(no_global_oom_handling))]
686    #[unstable(feature = "new_zeroed_alloc", issue = "129396")]
687    #[must_use]
688    pub fn new_zeroed_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
689        unsafe { RawVec::with_capacity_zeroed(len).into_box(len) }
690    }
691
692    /// Constructs a new boxed slice with uninitialized contents. Returns an error if
693    /// the allocation fails.
694    ///
695    /// # Examples
696    ///
697    /// ```
698    /// #![feature(allocator_api)]
699    ///
700    /// let mut values = Box::<[u32]>::try_new_uninit_slice(3)?;
701    /// // Deferred initialization:
702    /// values[0].write(1);
703    /// values[1].write(2);
704    /// values[2].write(3);
705    /// let values = unsafe { values.assume_init() };
706    ///
707    /// assert_eq!(*values, [1, 2, 3]);
708    /// # Ok::<(), std::alloc::AllocError>(())
709    /// ```
710    #[unstable(feature = "allocator_api", issue = "32838")]
711    #[inline]
712    pub fn try_new_uninit_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
713        let ptr = if T::IS_ZST || len == 0 {
714            NonNull::dangling()
715        } else {
716            let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
717                Ok(l) => l,
718                Err(_) => return Err(AllocError),
719            };
720            Global.allocate(layout)?.cast()
721        };
722        unsafe { Ok(RawVec::from_raw_parts_in(ptr.as_ptr(), len, Global).into_box(len)) }
723    }
724
725    /// Constructs a new boxed slice with uninitialized contents, with the memory
726    /// being filled with `0` bytes. Returns an error if the allocation fails.
727    ///
728    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
729    /// of this method.
730    ///
731    /// # Examples
732    ///
733    /// ```
734    /// #![feature(allocator_api)]
735    ///
736    /// let values = Box::<[u32]>::try_new_zeroed_slice(3)?;
737    /// let values = unsafe { values.assume_init() };
738    ///
739    /// assert_eq!(*values, [0, 0, 0]);
740    /// # Ok::<(), std::alloc::AllocError>(())
741    /// ```
742    ///
743    /// [zeroed]: mem::MaybeUninit::zeroed
744    #[unstable(feature = "allocator_api", issue = "32838")]
745    #[inline]
746    pub fn try_new_zeroed_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
747        let ptr = if T::IS_ZST || len == 0 {
748            NonNull::dangling()
749        } else {
750            let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
751                Ok(l) => l,
752                Err(_) => return Err(AllocError),
753            };
754            Global.allocate_zeroed(layout)?.cast()
755        };
756        unsafe { Ok(RawVec::from_raw_parts_in(ptr.as_ptr(), len, Global).into_box(len)) }
757    }
758
759    /// Converts the boxed slice into a boxed array.
760    ///
761    /// This operation does not reallocate; the underlying array of the slice is simply reinterpreted as an array type.
762    ///
763    /// If `N` is not exactly equal to the length of `self`, then this method returns `None`.
764    #[unstable(feature = "slice_as_array", issue = "133508")]
765    #[inline]
766    #[must_use]
767    pub fn into_array<const N: usize>(self) -> Option<Box<[T; N]>> {
768        if self.len() == N {
769            let ptr = Self::into_raw(self) as *mut [T; N];
770
771            // SAFETY: The underlying array of a slice has the exact same layout as an actual array `[T; N]` if `N` is equal to the slice's length.
772            let me = unsafe { Box::from_raw(ptr) };
773            Some(me)
774        } else {
775            None
776        }
777    }
778}
779
780impl<T, A: Allocator> Box<[T], A> {
781    /// Constructs a new boxed slice with uninitialized contents in the provided allocator.
782    ///
783    /// # Examples
784    ///
785    /// ```
786    /// #![feature(allocator_api)]
787    ///
788    /// use std::alloc::System;
789    ///
790    /// let mut values = Box::<[u32], _>::new_uninit_slice_in(3, System);
791    /// // Deferred initialization:
792    /// values[0].write(1);
793    /// values[1].write(2);
794    /// values[2].write(3);
795    /// let values = unsafe { values.assume_init() };
796    ///
797    /// assert_eq!(*values, [1, 2, 3])
798    /// ```
799    #[cfg(not(no_global_oom_handling))]
800    #[unstable(feature = "allocator_api", issue = "32838")]
801    // #[unstable(feature = "new_uninit", issue = "63291")]
802    #[must_use]
803    pub fn new_uninit_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
804        unsafe { RawVec::with_capacity_in(len, alloc).into_box(len) }
805    }
806
807    /// Constructs a new boxed slice with uninitialized contents in the provided allocator,
808    /// with the memory being filled with `0` bytes.
809    ///
810    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
811    /// of this method.
812    ///
813    /// # Examples
814    ///
815    /// ```
816    /// #![feature(allocator_api)]
817    ///
818    /// use std::alloc::System;
819    ///
820    /// let values = Box::<[u32], _>::new_zeroed_slice_in(3, System);
821    /// let values = unsafe { values.assume_init() };
822    ///
823    /// assert_eq!(*values, [0, 0, 0])
824    /// ```
825    ///
826    /// [zeroed]: mem::MaybeUninit::zeroed
827    #[cfg(not(no_global_oom_handling))]
828    #[unstable(feature = "allocator_api", issue = "32838")]
829    // #[unstable(feature = "new_uninit", issue = "63291")]
830    #[must_use]
831    pub fn new_zeroed_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
832        unsafe { RawVec::with_capacity_zeroed_in(len, alloc).into_box(len) }
833    }
834
835    /// Constructs a new boxed slice with uninitialized contents in the provided allocator. Returns an error if
836    /// the allocation fails.
837    ///
838    /// # Examples
839    ///
840    /// ```
841    /// #![feature(allocator_api)]
842    ///
843    /// use std::alloc::System;
844    ///
845    /// let mut values = Box::<[u32], _>::try_new_uninit_slice_in(3, System)?;
846    /// // Deferred initialization:
847    /// values[0].write(1);
848    /// values[1].write(2);
849    /// values[2].write(3);
850    /// let values = unsafe { values.assume_init() };
851    ///
852    /// assert_eq!(*values, [1, 2, 3]);
853    /// # Ok::<(), std::alloc::AllocError>(())
854    /// ```
855    #[unstable(feature = "allocator_api", issue = "32838")]
856    #[inline]
857    pub fn try_new_uninit_slice_in(
858        len: usize,
859        alloc: A,
860    ) -> Result<Box<[mem::MaybeUninit<T>], A>, AllocError> {
861        let ptr = if T::IS_ZST || len == 0 {
862            NonNull::dangling()
863        } else {
864            let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
865                Ok(l) => l,
866                Err(_) => return Err(AllocError),
867            };
868            alloc.allocate(layout)?.cast()
869        };
870        unsafe { Ok(RawVec::from_raw_parts_in(ptr.as_ptr(), len, alloc).into_box(len)) }
871    }
872
873    /// Constructs a new boxed slice with uninitialized contents in the provided allocator, with the memory
874    /// being filled with `0` bytes. Returns an error if the allocation fails.
875    ///
876    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
877    /// of this method.
878    ///
879    /// # Examples
880    ///
881    /// ```
882    /// #![feature(allocator_api)]
883    ///
884    /// use std::alloc::System;
885    ///
886    /// let values = Box::<[u32], _>::try_new_zeroed_slice_in(3, System)?;
887    /// let values = unsafe { values.assume_init() };
888    ///
889    /// assert_eq!(*values, [0, 0, 0]);
890    /// # Ok::<(), std::alloc::AllocError>(())
891    /// ```
892    ///
893    /// [zeroed]: mem::MaybeUninit::zeroed
894    #[unstable(feature = "allocator_api", issue = "32838")]
895    #[inline]
896    pub fn try_new_zeroed_slice_in(
897        len: usize,
898        alloc: A,
899    ) -> Result<Box<[mem::MaybeUninit<T>], A>, AllocError> {
900        let ptr = if T::IS_ZST || len == 0 {
901            NonNull::dangling()
902        } else {
903            let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
904                Ok(l) => l,
905                Err(_) => return Err(AllocError),
906            };
907            alloc.allocate_zeroed(layout)?.cast()
908        };
909        unsafe { Ok(RawVec::from_raw_parts_in(ptr.as_ptr(), len, alloc).into_box(len)) }
910    }
911}
912
913impl<T, A: Allocator> Box<mem::MaybeUninit<T>, A> {
914    /// Converts to `Box<T, A>`.
915    ///
916    /// # Safety
917    ///
918    /// As with [`MaybeUninit::assume_init`],
919    /// it is up to the caller to guarantee that the value
920    /// really is in an initialized state.
921    /// Calling this when the content is not yet fully initialized
922    /// causes immediate undefined behavior.
923    ///
924    /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
925    ///
926    /// # Examples
927    ///
928    /// ```
929    /// let mut five = Box::<u32>::new_uninit();
930    /// // Deferred initialization:
931    /// five.write(5);
932    /// let five: Box<u32> = unsafe { five.assume_init() };
933    ///
934    /// assert_eq!(*five, 5)
935    /// ```
936    #[stable(feature = "new_uninit", since = "1.82.0")]
937    #[inline]
938    pub unsafe fn assume_init(self) -> Box<T, A> {
939        let (raw, alloc) = Box::into_raw_with_allocator(self);
940        unsafe { Box::from_raw_in(raw as *mut T, alloc) }
941    }
942
943    /// Writes the value and converts to `Box<T, A>`.
944    ///
945    /// This method converts the box similarly to [`Box::assume_init`] but
946    /// writes `value` into it before conversion thus guaranteeing safety.
947    /// In some scenarios use of this method may improve performance because
948    /// the compiler may be able to optimize copying from stack.
949    ///
950    /// # Examples
951    ///
952    /// ```
953    /// #![feature(box_uninit_write)]
954    ///
955    /// let big_box = Box::<[usize; 1024]>::new_uninit();
956    ///
957    /// let mut array = [0; 1024];
958    /// for (i, place) in array.iter_mut().enumerate() {
959    ///     *place = i;
960    /// }
961    ///
962    /// // The optimizer may be able to elide this copy, so previous code writes
963    /// // to heap directly.
964    /// let big_box = Box::write(big_box, array);
965    ///
966    /// for (i, x) in big_box.iter().enumerate() {
967    ///     assert_eq!(*x, i);
968    /// }
969    /// ```
970    #[unstable(feature = "box_uninit_write", issue = "129397")]
971    #[inline]
972    pub fn write(mut boxed: Self, value: T) -> Box<T, A> {
973        unsafe {
974            (*boxed).write(value);
975            boxed.assume_init()
976        }
977    }
978}
979
980impl<T, A: Allocator> Box<[mem::MaybeUninit<T>], A> {
981    /// Converts to `Box<[T], A>`.
982    ///
983    /// # Safety
984    ///
985    /// As with [`MaybeUninit::assume_init`],
986    /// it is up to the caller to guarantee that the values
987    /// really are in an initialized state.
988    /// Calling this when the content is not yet fully initialized
989    /// causes immediate undefined behavior.
990    ///
991    /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
992    ///
993    /// # Examples
994    ///
995    /// ```
996    /// let mut values = Box::<[u32]>::new_uninit_slice(3);
997    /// // Deferred initialization:
998    /// values[0].write(1);
999    /// values[1].write(2);
1000    /// values[2].write(3);
1001    /// let values = unsafe { values.assume_init() };
1002    ///
1003    /// assert_eq!(*values, [1, 2, 3])
1004    /// ```
1005    #[stable(feature = "new_uninit", since = "1.82.0")]
1006    #[inline]
1007    pub unsafe fn assume_init(self) -> Box<[T], A> {
1008        let (raw, alloc) = Box::into_raw_with_allocator(self);
1009        unsafe { Box::from_raw_in(raw as *mut [T], alloc) }
1010    }
1011}
1012
1013impl<T: ?Sized> Box<T> {
1014    /// Constructs a box from a raw pointer.
1015    ///
1016    /// After calling this function, the raw pointer is owned by the
1017    /// resulting `Box`. Specifically, the `Box` destructor will call
1018    /// the destructor of `T` and free the allocated memory. For this
1019    /// to be safe, the memory must have been allocated in accordance
1020    /// with the [memory layout] used by `Box` .
1021    ///
1022    /// # Safety
1023    ///
1024    /// This function is unsafe because improper use may lead to
1025    /// memory problems. For example, a double-free may occur if the
1026    /// function is called twice on the same raw pointer.
1027    ///
1028    /// The raw pointer must point to a block of memory allocated by the global allocator.
1029    ///
1030    /// The safety conditions are described in the [memory layout] section.
1031    ///
1032    /// # Examples
1033    ///
1034    /// Recreate a `Box` which was previously converted to a raw pointer
1035    /// using [`Box::into_raw`]:
1036    /// ```
1037    /// let x = Box::new(5);
1038    /// let ptr = Box::into_raw(x);
1039    /// let x = unsafe { Box::from_raw(ptr) };
1040    /// ```
1041    /// Manually create a `Box` from scratch by using the global allocator:
1042    /// ```
1043    /// use std::alloc::{alloc, Layout};
1044    ///
1045    /// unsafe {
1046    ///     let ptr = alloc(Layout::new::<i32>()) as *mut i32;
1047    ///     // In general .write is required to avoid attempting to destruct
1048    ///     // the (uninitialized) previous contents of `ptr`, though for this
1049    ///     // simple example `*ptr = 5` would have worked as well.
1050    ///     ptr.write(5);
1051    ///     let x = Box::from_raw(ptr);
1052    /// }
1053    /// ```
1054    ///
1055    /// [memory layout]: self#memory-layout
1056    /// [`Layout`]: crate::Layout
1057    #[stable(feature = "box_raw", since = "1.4.0")]
1058    #[inline]
1059    #[must_use = "call `drop(Box::from_raw(ptr))` if you intend to drop the `Box`"]
1060    pub unsafe fn from_raw(raw: *mut T) -> Self {
1061        unsafe { Self::from_raw_in(raw, Global) }
1062    }
1063
1064    /// Constructs a box from a `NonNull` pointer.
1065    ///
1066    /// After calling this function, the `NonNull` pointer is owned by
1067    /// the resulting `Box`. Specifically, the `Box` destructor will call
1068    /// the destructor of `T` and free the allocated memory. For this
1069    /// to be safe, the memory must have been allocated in accordance
1070    /// with the [memory layout] used by `Box` .
1071    ///
1072    /// # Safety
1073    ///
1074    /// This function is unsafe because improper use may lead to
1075    /// memory problems. For example, a double-free may occur if the
1076    /// function is called twice on the same `NonNull` pointer.
1077    ///
1078    /// The non-null pointer must point to a block of memory allocated by the global allocator.
1079    ///
1080    /// The safety conditions are described in the [memory layout] section.
1081    ///
1082    /// # Examples
1083    ///
1084    /// Recreate a `Box` which was previously converted to a `NonNull`
1085    /// pointer using [`Box::into_non_null`]:
1086    /// ```
1087    /// #![feature(box_vec_non_null)]
1088    ///
1089    /// let x = Box::new(5);
1090    /// let non_null = Box::into_non_null(x);
1091    /// let x = unsafe { Box::from_non_null(non_null) };
1092    /// ```
1093    /// Manually create a `Box` from scratch by using the global allocator:
1094    /// ```
1095    /// #![feature(box_vec_non_null)]
1096    ///
1097    /// use std::alloc::{alloc, Layout};
1098    /// use std::ptr::NonNull;
1099    ///
1100    /// unsafe {
1101    ///     let non_null = NonNull::new(alloc(Layout::new::<i32>()).cast::<i32>())
1102    ///         .expect("allocation failed");
1103    ///     // In general .write is required to avoid attempting to destruct
1104    ///     // the (uninitialized) previous contents of `non_null`.
1105    ///     non_null.write(5);
1106    ///     let x = Box::from_non_null(non_null);
1107    /// }
1108    /// ```
1109    ///
1110    /// [memory layout]: self#memory-layout
1111    /// [`Layout`]: crate::Layout
1112    #[unstable(feature = "box_vec_non_null", reason = "new API", issue = "130364")]
1113    #[inline]
1114    #[must_use = "call `drop(Box::from_non_null(ptr))` if you intend to drop the `Box`"]
1115    pub unsafe fn from_non_null(ptr: NonNull<T>) -> Self {
1116        unsafe { Self::from_raw(ptr.as_ptr()) }
1117    }
1118}
1119
1120impl<T: ?Sized, A: Allocator> Box<T, A> {
1121    /// Constructs a box from a raw pointer in the given allocator.
1122    ///
1123    /// After calling this function, the raw pointer is owned by the
1124    /// resulting `Box`. Specifically, the `Box` destructor will call
1125    /// the destructor of `T` and free the allocated memory. For this
1126    /// to be safe, the memory must have been allocated in accordance
1127    /// with the [memory layout] used by `Box` .
1128    ///
1129    /// # Safety
1130    ///
1131    /// This function is unsafe because improper use may lead to
1132    /// memory problems. For example, a double-free may occur if the
1133    /// function is called twice on the same raw pointer.
1134    ///
1135    /// The raw pointer must point to a block of memory allocated by `alloc`.
1136    ///
1137    /// # Examples
1138    ///
1139    /// Recreate a `Box` which was previously converted to a raw pointer
1140    /// using [`Box::into_raw_with_allocator`]:
1141    /// ```
1142    /// #![feature(allocator_api)]
1143    ///
1144    /// use std::alloc::System;
1145    ///
1146    /// let x = Box::new_in(5, System);
1147    /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
1148    /// let x = unsafe { Box::from_raw_in(ptr, alloc) };
1149    /// ```
1150    /// Manually create a `Box` from scratch by using the system allocator:
1151    /// ```
1152    /// #![feature(allocator_api, slice_ptr_get)]
1153    ///
1154    /// use std::alloc::{Allocator, Layout, System};
1155    ///
1156    /// unsafe {
1157    ///     let ptr = System.allocate(Layout::new::<i32>())?.as_mut_ptr() as *mut i32;
1158    ///     // In general .write is required to avoid attempting to destruct
1159    ///     // the (uninitialized) previous contents of `ptr`, though for this
1160    ///     // simple example `*ptr = 5` would have worked as well.
1161    ///     ptr.write(5);
1162    ///     let x = Box::from_raw_in(ptr, System);
1163    /// }
1164    /// # Ok::<(), std::alloc::AllocError>(())
1165    /// ```
1166    ///
1167    /// [memory layout]: self#memory-layout
1168    /// [`Layout`]: crate::Layout
1169    #[unstable(feature = "allocator_api", issue = "32838")]
1170    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1171    #[inline]
1172    pub const unsafe fn from_raw_in(raw: *mut T, alloc: A) -> Self {
1173        Box(unsafe { Unique::new_unchecked(raw) }, alloc)
1174    }
1175
1176    /// Constructs a box from a `NonNull` pointer in the given allocator.
1177    ///
1178    /// After calling this function, the `NonNull` pointer is owned by
1179    /// the resulting `Box`. Specifically, the `Box` destructor will call
1180    /// the destructor of `T` and free the allocated memory. For this
1181    /// to be safe, the memory must have been allocated in accordance
1182    /// with the [memory layout] used by `Box` .
1183    ///
1184    /// # Safety
1185    ///
1186    /// This function is unsafe because improper use may lead to
1187    /// memory problems. For example, a double-free may occur if the
1188    /// function is called twice on the same raw pointer.
1189    ///
1190    /// The non-null pointer must point to a block of memory allocated by `alloc`.
1191    ///
1192    /// # Examples
1193    ///
1194    /// Recreate a `Box` which was previously converted to a `NonNull` pointer
1195    /// using [`Box::into_non_null_with_allocator`]:
1196    /// ```
1197    /// #![feature(allocator_api, box_vec_non_null)]
1198    ///
1199    /// use std::alloc::System;
1200    ///
1201    /// let x = Box::new_in(5, System);
1202    /// let (non_null, alloc) = Box::into_non_null_with_allocator(x);
1203    /// let x = unsafe { Box::from_non_null_in(non_null, alloc) };
1204    /// ```
1205    /// Manually create a `Box` from scratch by using the system allocator:
1206    /// ```
1207    /// #![feature(allocator_api, box_vec_non_null, slice_ptr_get)]
1208    ///
1209    /// use std::alloc::{Allocator, Layout, System};
1210    ///
1211    /// unsafe {
1212    ///     let non_null = System.allocate(Layout::new::<i32>())?.cast::<i32>();
1213    ///     // In general .write is required to avoid attempting to destruct
1214    ///     // the (uninitialized) previous contents of `non_null`.
1215    ///     non_null.write(5);
1216    ///     let x = Box::from_non_null_in(non_null, System);
1217    /// }
1218    /// # Ok::<(), std::alloc::AllocError>(())
1219    /// ```
1220    ///
1221    /// [memory layout]: self#memory-layout
1222    /// [`Layout`]: crate::Layout
1223    #[unstable(feature = "allocator_api", issue = "32838")]
1224    // #[unstable(feature = "box_vec_non_null", reason = "new API", issue = "130364")]
1225    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1226    #[inline]
1227    pub const unsafe fn from_non_null_in(raw: NonNull<T>, alloc: A) -> Self {
1228        // SAFETY: guaranteed by the caller.
1229        unsafe { Box::from_raw_in(raw.as_ptr(), alloc) }
1230    }
1231
1232    /// Consumes the `Box`, returning a wrapped raw pointer.
1233    ///
1234    /// The pointer will be properly aligned and non-null.
1235    ///
1236    /// After calling this function, the caller is responsible for the
1237    /// memory previously managed by the `Box`. In particular, the
1238    /// caller should properly destroy `T` and release the memory, taking
1239    /// into account the [memory layout] used by `Box`. The easiest way to
1240    /// do this is to convert the raw pointer back into a `Box` with the
1241    /// [`Box::from_raw`] function, allowing the `Box` destructor to perform
1242    /// the cleanup.
1243    ///
1244    /// Note: this is an associated function, which means that you have
1245    /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
1246    /// is so that there is no conflict with a method on the inner type.
1247    ///
1248    /// # Examples
1249    /// Converting the raw pointer back into a `Box` with [`Box::from_raw`]
1250    /// for automatic cleanup:
1251    /// ```
1252    /// let x = Box::new(String::from("Hello"));
1253    /// let ptr = Box::into_raw(x);
1254    /// let x = unsafe { Box::from_raw(ptr) };
1255    /// ```
1256    /// Manual cleanup by explicitly running the destructor and deallocating
1257    /// the memory:
1258    /// ```
1259    /// use std::alloc::{dealloc, Layout};
1260    /// use std::ptr;
1261    ///
1262    /// let x = Box::new(String::from("Hello"));
1263    /// let ptr = Box::into_raw(x);
1264    /// unsafe {
1265    ///     ptr::drop_in_place(ptr);
1266    ///     dealloc(ptr as *mut u8, Layout::new::<String>());
1267    /// }
1268    /// ```
1269    /// Note: This is equivalent to the following:
1270    /// ```
1271    /// let x = Box::new(String::from("Hello"));
1272    /// let ptr = Box::into_raw(x);
1273    /// unsafe {
1274    ///     drop(Box::from_raw(ptr));
1275    /// }
1276    /// ```
1277    ///
1278    /// [memory layout]: self#memory-layout
1279    #[must_use = "losing the pointer will leak memory"]
1280    #[stable(feature = "box_raw", since = "1.4.0")]
1281    #[inline]
1282    pub fn into_raw(b: Self) -> *mut T {
1283        // Make sure Miri realizes that we transition from a noalias pointer to a raw pointer here.
1284        unsafe { &raw mut *&mut *Self::into_raw_with_allocator(b).0 }
1285    }
1286
1287    /// Consumes the `Box`, returning a wrapped `NonNull` pointer.
1288    ///
1289    /// The pointer will be properly aligned.
1290    ///
1291    /// After calling this function, the caller is responsible for the
1292    /// memory previously managed by the `Box`. In particular, the
1293    /// caller should properly destroy `T` and release the memory, taking
1294    /// into account the [memory layout] used by `Box`. The easiest way to
1295    /// do this is to convert the `NonNull` pointer back into a `Box` with the
1296    /// [`Box::from_non_null`] function, allowing the `Box` destructor to
1297    /// perform the cleanup.
1298    ///
1299    /// Note: this is an associated function, which means that you have
1300    /// to call it as `Box::into_non_null(b)` instead of `b.into_non_null()`.
1301    /// This is so that there is no conflict with a method on the inner type.
1302    ///
1303    /// # Examples
1304    /// Converting the `NonNull` pointer back into a `Box` with [`Box::from_non_null`]
1305    /// for automatic cleanup:
1306    /// ```
1307    /// #![feature(box_vec_non_null)]
1308    ///
1309    /// let x = Box::new(String::from("Hello"));
1310    /// let non_null = Box::into_non_null(x);
1311    /// let x = unsafe { Box::from_non_null(non_null) };
1312    /// ```
1313    /// Manual cleanup by explicitly running the destructor and deallocating
1314    /// the memory:
1315    /// ```
1316    /// #![feature(box_vec_non_null)]
1317    ///
1318    /// use std::alloc::{dealloc, Layout};
1319    ///
1320    /// let x = Box::new(String::from("Hello"));
1321    /// let non_null = Box::into_non_null(x);
1322    /// unsafe {
1323    ///     non_null.drop_in_place();
1324    ///     dealloc(non_null.as_ptr().cast::<u8>(), Layout::new::<String>());
1325    /// }
1326    /// ```
1327    /// Note: This is equivalent to the following:
1328    /// ```
1329    /// #![feature(box_vec_non_null)]
1330    ///
1331    /// let x = Box::new(String::from("Hello"));
1332    /// let non_null = Box::into_non_null(x);
1333    /// unsafe {
1334    ///     drop(Box::from_non_null(non_null));
1335    /// }
1336    /// ```
1337    ///
1338    /// [memory layout]: self#memory-layout
1339    #[must_use = "losing the pointer will leak memory"]
1340    #[unstable(feature = "box_vec_non_null", reason = "new API", issue = "130364")]
1341    #[inline]
1342    pub fn into_non_null(b: Self) -> NonNull<T> {
1343        // SAFETY: `Box` is guaranteed to be non-null.
1344        unsafe { NonNull::new_unchecked(Self::into_raw(b)) }
1345    }
1346
1347    /// Consumes the `Box`, returning a wrapped raw pointer and the allocator.
1348    ///
1349    /// The pointer will be properly aligned and non-null.
1350    ///
1351    /// After calling this function, the caller is responsible for the
1352    /// memory previously managed by the `Box`. In particular, the
1353    /// caller should properly destroy `T` and release the memory, taking
1354    /// into account the [memory layout] used by `Box`. The easiest way to
1355    /// do this is to convert the raw pointer back into a `Box` with the
1356    /// [`Box::from_raw_in`] function, allowing the `Box` destructor to perform
1357    /// the cleanup.
1358    ///
1359    /// Note: this is an associated function, which means that you have
1360    /// to call it as `Box::into_raw_with_allocator(b)` instead of `b.into_raw_with_allocator()`. This
1361    /// is so that there is no conflict with a method on the inner type.
1362    ///
1363    /// # Examples
1364    /// Converting the raw pointer back into a `Box` with [`Box::from_raw_in`]
1365    /// for automatic cleanup:
1366    /// ```
1367    /// #![feature(allocator_api)]
1368    ///
1369    /// use std::alloc::System;
1370    ///
1371    /// let x = Box::new_in(String::from("Hello"), System);
1372    /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
1373    /// let x = unsafe { Box::from_raw_in(ptr, alloc) };
1374    /// ```
1375    /// Manual cleanup by explicitly running the destructor and deallocating
1376    /// the memory:
1377    /// ```
1378    /// #![feature(allocator_api)]
1379    ///
1380    /// use std::alloc::{Allocator, Layout, System};
1381    /// use std::ptr::{self, NonNull};
1382    ///
1383    /// let x = Box::new_in(String::from("Hello"), System);
1384    /// let (ptr, alloc) = Box::into_raw_with_allocator(x);
1385    /// unsafe {
1386    ///     ptr::drop_in_place(ptr);
1387    ///     let non_null = NonNull::new_unchecked(ptr);
1388    ///     alloc.deallocate(non_null.cast(), Layout::new::<String>());
1389    /// }
1390    /// ```
1391    ///
1392    /// [memory layout]: self#memory-layout
1393    #[must_use = "losing the pointer will leak memory"]
1394    #[unstable(feature = "allocator_api", issue = "32838")]
1395    #[inline]
1396    pub fn into_raw_with_allocator(b: Self) -> (*mut T, A) {
1397        let mut b = mem::ManuallyDrop::new(b);
1398        // We carefully get the raw pointer out in a way that Miri's aliasing model understands what
1399        // is happening: using the primitive "deref" of `Box`. In case `A` is *not* `Global`, we
1400        // want *no* aliasing requirements here!
1401        // In case `A` *is* `Global`, this does not quite have the right behavior; `into_raw`
1402        // works around that.
1403        let ptr = &raw mut **b;
1404        let alloc = unsafe { ptr::read(&b.1) };
1405        (ptr, alloc)
1406    }
1407
1408    /// Consumes the `Box`, returning a wrapped `NonNull` pointer and the allocator.
1409    ///
1410    /// The pointer will be properly aligned.
1411    ///
1412    /// After calling this function, the caller is responsible for the
1413    /// memory previously managed by the `Box`. In particular, the
1414    /// caller should properly destroy `T` and release the memory, taking
1415    /// into account the [memory layout] used by `Box`. The easiest way to
1416    /// do this is to convert the `NonNull` pointer back into a `Box` with the
1417    /// [`Box::from_non_null_in`] function, allowing the `Box` destructor to
1418    /// perform the cleanup.
1419    ///
1420    /// Note: this is an associated function, which means that you have
1421    /// to call it as `Box::into_non_null_with_allocator(b)` instead of
1422    /// `b.into_non_null_with_allocator()`. This is so that there is no
1423    /// conflict with a method on the inner type.
1424    ///
1425    /// # Examples
1426    /// Converting the `NonNull` pointer back into a `Box` with
1427    /// [`Box::from_non_null_in`] for automatic cleanup:
1428    /// ```
1429    /// #![feature(allocator_api, box_vec_non_null)]
1430    ///
1431    /// use std::alloc::System;
1432    ///
1433    /// let x = Box::new_in(String::from("Hello"), System);
1434    /// let (non_null, alloc) = Box::into_non_null_with_allocator(x);
1435    /// let x = unsafe { Box::from_non_null_in(non_null, alloc) };
1436    /// ```
1437    /// Manual cleanup by explicitly running the destructor and deallocating
1438    /// the memory:
1439    /// ```
1440    /// #![feature(allocator_api, box_vec_non_null)]
1441    ///
1442    /// use std::alloc::{Allocator, Layout, System};
1443    ///
1444    /// let x = Box::new_in(String::from("Hello"), System);
1445    /// let (non_null, alloc) = Box::into_non_null_with_allocator(x);
1446    /// unsafe {
1447    ///     non_null.drop_in_place();
1448    ///     alloc.deallocate(non_null.cast::<u8>(), Layout::new::<String>());
1449    /// }
1450    /// ```
1451    ///
1452    /// [memory layout]: self#memory-layout
1453    #[must_use = "losing the pointer will leak memory"]
1454    #[unstable(feature = "allocator_api", issue = "32838")]
1455    // #[unstable(feature = "box_vec_non_null", reason = "new API", issue = "130364")]
1456    #[inline]
1457    pub fn into_non_null_with_allocator(b: Self) -> (NonNull<T>, A) {
1458        let (ptr, alloc) = Box::into_raw_with_allocator(b);
1459        // SAFETY: `Box` is guaranteed to be non-null.
1460        unsafe { (NonNull::new_unchecked(ptr), alloc) }
1461    }
1462
1463    #[unstable(
1464        feature = "ptr_internals",
1465        issue = "none",
1466        reason = "use `Box::leak(b).into()` or `Unique::from(Box::leak(b))` instead"
1467    )]
1468    #[inline]
1469    #[doc(hidden)]
1470    pub fn into_unique(b: Self) -> (Unique<T>, A) {
1471        let (ptr, alloc) = Box::into_raw_with_allocator(b);
1472        unsafe { (Unique::from(&mut *ptr), alloc) }
1473    }
1474
1475    /// Returns a raw mutable pointer to the `Box`'s contents.
1476    ///
1477    /// The caller must ensure that the `Box` outlives the pointer this
1478    /// function returns, or else it will end up dangling.
1479    ///
1480    /// This method guarantees that for the purpose of the aliasing model, this method
1481    /// does not materialize a reference to the underlying memory, and thus the returned pointer
1482    /// will remain valid when mixed with other calls to [`as_ptr`] and [`as_mut_ptr`].
1483    /// Note that calling other methods that materialize references to the memory
1484    /// may still invalidate this pointer.
1485    /// See the example below for how this guarantee can be used.
1486    ///
1487    /// # Examples
1488    ///
1489    /// Due to the aliasing guarantee, the following code is legal:
1490    ///
1491    /// ```rust
1492    /// #![feature(box_as_ptr)]
1493    ///
1494    /// unsafe {
1495    ///     let mut b = Box::new(0);
1496    ///     let ptr1 = Box::as_mut_ptr(&mut b);
1497    ///     ptr1.write(1);
1498    ///     let ptr2 = Box::as_mut_ptr(&mut b);
1499    ///     ptr2.write(2);
1500    ///     // Notably, the write to `ptr2` did *not* invalidate `ptr1`:
1501    ///     ptr1.write(3);
1502    /// }
1503    /// ```
1504    ///
1505    /// [`as_mut_ptr`]: Self::as_mut_ptr
1506    /// [`as_ptr`]: Self::as_ptr
1507    #[unstable(feature = "box_as_ptr", issue = "129090")]
1508    #[rustc_never_returns_null_ptr]
1509    #[rustc_as_ptr]
1510    #[inline]
1511    pub fn as_mut_ptr(b: &mut Self) -> *mut T {
1512        // This is a primitive deref, not going through `DerefMut`, and therefore not materializing
1513        // any references.
1514        &raw mut **b
1515    }
1516
1517    /// Returns a raw pointer to the `Box`'s contents.
1518    ///
1519    /// The caller must ensure that the `Box` outlives the pointer this
1520    /// function returns, or else it will end up dangling.
1521    ///
1522    /// The caller must also ensure that the memory the pointer (non-transitively) points to
1523    /// is never written to (except inside an `UnsafeCell`) using this pointer or any pointer
1524    /// derived from it. If you need to mutate the contents of the `Box`, use [`as_mut_ptr`].
1525    ///
1526    /// This method guarantees that for the purpose of the aliasing model, this method
1527    /// does not materialize a reference to the underlying memory, and thus the returned pointer
1528    /// will remain valid when mixed with other calls to [`as_ptr`] and [`as_mut_ptr`].
1529    /// Note that calling other methods that materialize mutable references to the memory,
1530    /// as well as writing to this memory, may still invalidate this pointer.
1531    /// See the example below for how this guarantee can be used.
1532    ///
1533    /// # Examples
1534    ///
1535    /// Due to the aliasing guarantee, the following code is legal:
1536    ///
1537    /// ```rust
1538    /// #![feature(box_as_ptr)]
1539    ///
1540    /// unsafe {
1541    ///     let mut v = Box::new(0);
1542    ///     let ptr1 = Box::as_ptr(&v);
1543    ///     let ptr2 = Box::as_mut_ptr(&mut v);
1544    ///     let _val = ptr2.read();
1545    ///     // No write to this memory has happened yet, so `ptr1` is still valid.
1546    ///     let _val = ptr1.read();
1547    ///     // However, once we do a write...
1548    ///     ptr2.write(1);
1549    ///     // ... `ptr1` is no longer valid.
1550    ///     // This would be UB: let _val = ptr1.read();
1551    /// }
1552    /// ```
1553    ///
1554    /// [`as_mut_ptr`]: Self::as_mut_ptr
1555    /// [`as_ptr`]: Self::as_ptr
1556    #[unstable(feature = "box_as_ptr", issue = "129090")]
1557    #[rustc_never_returns_null_ptr]
1558    #[rustc_as_ptr]
1559    #[inline]
1560    pub fn as_ptr(b: &Self) -> *const T {
1561        // This is a primitive deref, not going through `DerefMut`, and therefore not materializing
1562        // any references.
1563        &raw const **b
1564    }
1565
1566    /// Returns a reference to the underlying allocator.
1567    ///
1568    /// Note: this is an associated function, which means that you have
1569    /// to call it as `Box::allocator(&b)` instead of `b.allocator()`. This
1570    /// is so that there is no conflict with a method on the inner type.
1571    #[unstable(feature = "allocator_api", issue = "32838")]
1572    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1573    #[inline]
1574    pub const fn allocator(b: &Self) -> &A {
1575        &b.1
1576    }
1577
1578    /// Consumes and leaks the `Box`, returning a mutable reference,
1579    /// `&'a mut T`.
1580    ///
1581    /// Note that the type `T` must outlive the chosen lifetime `'a`. If the type
1582    /// has only static references, or none at all, then this may be chosen to be
1583    /// `'static`.
1584    ///
1585    /// This function is mainly useful for data that lives for the remainder of
1586    /// the program's life. Dropping the returned reference will cause a memory
1587    /// leak. If this is not acceptable, the reference should first be wrapped
1588    /// with the [`Box::from_raw`] function producing a `Box`. This `Box` can
1589    /// then be dropped which will properly destroy `T` and release the
1590    /// allocated memory.
1591    ///
1592    /// Note: this is an associated function, which means that you have
1593    /// to call it as `Box::leak(b)` instead of `b.leak()`. This
1594    /// is so that there is no conflict with a method on the inner type.
1595    ///
1596    /// # Examples
1597    ///
1598    /// Simple usage:
1599    ///
1600    /// ```
1601    /// let x = Box::new(41);
1602    /// let static_ref: &'static mut usize = Box::leak(x);
1603    /// *static_ref += 1;
1604    /// assert_eq!(*static_ref, 42);
1605    /// # // FIXME(https://github.com/rust-lang/miri/issues/3670):
1606    /// # // use -Zmiri-disable-leak-check instead of unleaking in tests meant to leak.
1607    /// # drop(unsafe { Box::from_raw(static_ref) });
1608    /// ```
1609    ///
1610    /// Unsized data:
1611    ///
1612    /// ```
1613    /// let x = vec![1, 2, 3].into_boxed_slice();
1614    /// let static_ref = Box::leak(x);
1615    /// static_ref[0] = 4;
1616    /// assert_eq!(*static_ref, [4, 2, 3]);
1617    /// # // FIXME(https://github.com/rust-lang/miri/issues/3670):
1618    /// # // use -Zmiri-disable-leak-check instead of unleaking in tests meant to leak.
1619    /// # drop(unsafe { Box::from_raw(static_ref) });
1620    /// ```
1621    #[stable(feature = "box_leak", since = "1.26.0")]
1622    #[inline]
1623    pub fn leak<'a>(b: Self) -> &'a mut T
1624    where
1625        A: 'a,
1626    {
1627        unsafe { &mut *Box::into_raw(b) }
1628    }
1629
1630    /// Converts a `Box<T>` into a `Pin<Box<T>>`. If `T` does not implement [`Unpin`], then
1631    /// `*boxed` will be pinned in memory and unable to be moved.
1632    ///
1633    /// This conversion does not allocate on the heap and happens in place.
1634    ///
1635    /// This is also available via [`From`].
1636    ///
1637    /// Constructing and pinning a `Box` with <code>Box::into_pin([Box::new]\(x))</code>
1638    /// can also be written more concisely using <code>[Box::pin]\(x)</code>.
1639    /// This `into_pin` method is useful if you already have a `Box<T>`, or you are
1640    /// constructing a (pinned) `Box` in a different way than with [`Box::new`].
1641    ///
1642    /// # Notes
1643    ///
1644    /// It's not recommended that crates add an impl like `From<Box<T>> for Pin<T>`,
1645    /// as it'll introduce an ambiguity when calling `Pin::from`.
1646    /// A demonstration of such a poor impl is shown below.
1647    ///
1648    /// ```compile_fail
1649    /// # use std::pin::Pin;
1650    /// struct Foo; // A type defined in this crate.
1651    /// impl From<Box<()>> for Pin<Foo> {
1652    ///     fn from(_: Box<()>) -> Pin<Foo> {
1653    ///         Pin::new(Foo)
1654    ///     }
1655    /// }
1656    ///
1657    /// let foo = Box::new(());
1658    /// let bar = Pin::from(foo);
1659    /// ```
1660    #[stable(feature = "box_into_pin", since = "1.63.0")]
1661    #[rustc_const_unstable(feature = "const_box", issue = "92521")]
1662    pub const fn into_pin(boxed: Self) -> Pin<Self>
1663    where
1664        A: 'static,
1665    {
1666        // It's not possible to move or replace the insides of a `Pin<Box<T>>`
1667        // when `T: !Unpin`, so it's safe to pin it directly without any
1668        // additional requirements.
1669        unsafe { Pin::new_unchecked(boxed) }
1670    }
1671}
1672
1673#[stable(feature = "rust1", since = "1.0.0")]
1674unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Box<T, A> {
1675    #[inline]
1676    fn drop(&mut self) {
1677        // the T in the Box is dropped by the compiler before the destructor is run
1678
1679        let ptr = self.0;
1680
1681        unsafe {
1682            let layout = Layout::for_value_raw(ptr.as_ptr());
1683            if layout.size() != 0 {
1684                self.1.deallocate(From::from(ptr.cast()), layout);
1685            }
1686        }
1687    }
1688}
1689
1690#[cfg(not(no_global_oom_handling))]
1691#[stable(feature = "rust1", since = "1.0.0")]
1692impl<T: Default> Default for Box<T> {
1693    /// Creates a `Box<T>`, with the `Default` value for T.
1694    #[inline]
1695    fn default() -> Self {
1696        Box::write(Box::new_uninit(), T::default())
1697    }
1698}
1699
1700#[cfg(not(no_global_oom_handling))]
1701#[stable(feature = "rust1", since = "1.0.0")]
1702impl<T> Default for Box<[T]> {
1703    #[inline]
1704    fn default() -> Self {
1705        let ptr: Unique<[T]> = Unique::<[T; 0]>::dangling();
1706        Box(ptr, Global)
1707    }
1708}
1709
1710#[cfg(not(no_global_oom_handling))]
1711#[stable(feature = "default_box_extra", since = "1.17.0")]
1712impl Default for Box<str> {
1713    #[inline]
1714    fn default() -> Self {
1715        // SAFETY: This is the same as `Unique::cast<U>` but with an unsized `U = str`.
1716        let ptr: Unique<str> = unsafe {
1717            let bytes: Unique<[u8]> = Unique::<[u8; 0]>::dangling();
1718            Unique::new_unchecked(bytes.as_ptr() as *mut str)
1719        };
1720        Box(ptr, Global)
1721    }
1722}
1723
1724#[cfg(not(no_global_oom_handling))]
1725#[stable(feature = "rust1", since = "1.0.0")]
1726impl<T: Clone, A: Allocator + Clone> Clone for Box<T, A> {
1727    /// Returns a new box with a `clone()` of this box's contents.
1728    ///
1729    /// # Examples
1730    ///
1731    /// ```
1732    /// let x = Box::new(5);
1733    /// let y = x.clone();
1734    ///
1735    /// // The value is the same
1736    /// assert_eq!(x, y);
1737    ///
1738    /// // But they are unique objects
1739    /// assert_ne!(&*x as *const i32, &*y as *const i32);
1740    /// ```
1741    #[inline]
1742    fn clone(&self) -> Self {
1743        // Pre-allocate memory to allow writing the cloned value directly.
1744        let mut boxed = Self::new_uninit_in(self.1.clone());
1745        unsafe {
1746            (**self).clone_to_uninit(boxed.as_mut_ptr().cast());
1747            boxed.assume_init()
1748        }
1749    }
1750
1751    /// Copies `source`'s contents into `self` without creating a new allocation.
1752    ///
1753    /// # Examples
1754    ///
1755    /// ```
1756    /// let x = Box::new(5);
1757    /// let mut y = Box::new(10);
1758    /// let yp: *const i32 = &*y;
1759    ///
1760    /// y.clone_from(&x);
1761    ///
1762    /// // The value is the same
1763    /// assert_eq!(x, y);
1764    ///
1765    /// // And no allocation occurred
1766    /// assert_eq!(yp, &*y);
1767    /// ```
1768    #[inline]
1769    fn clone_from(&mut self, source: &Self) {
1770        (**self).clone_from(&(**source));
1771    }
1772}
1773
1774#[cfg(not(no_global_oom_handling))]
1775#[stable(feature = "box_slice_clone", since = "1.3.0")]
1776impl<T: Clone, A: Allocator + Clone> Clone for Box<[T], A> {
1777    fn clone(&self) -> Self {
1778        let alloc = Box::allocator(self).clone();
1779        self.to_vec_in(alloc).into_boxed_slice()
1780    }
1781
1782    /// Copies `source`'s contents into `self` without creating a new allocation,
1783    /// so long as the two are of the same length.
1784    ///
1785    /// # Examples
1786    ///
1787    /// ```
1788    /// let x = Box::new([5, 6, 7]);
1789    /// let mut y = Box::new([8, 9, 10]);
1790    /// let yp: *const [i32] = &*y;
1791    ///
1792    /// y.clone_from(&x);
1793    ///
1794    /// // The value is the same
1795    /// assert_eq!(x, y);
1796    ///
1797    /// // And no allocation occurred
1798    /// assert_eq!(yp, &*y);
1799    /// ```
1800    fn clone_from(&mut self, source: &Self) {
1801        if self.len() == source.len() {
1802            self.clone_from_slice(&source);
1803        } else {
1804            *self = source.clone();
1805        }
1806    }
1807}
1808
1809#[cfg(not(no_global_oom_handling))]
1810#[stable(feature = "box_slice_clone", since = "1.3.0")]
1811impl Clone for Box<str> {
1812    fn clone(&self) -> Self {
1813        // this makes a copy of the data
1814        let buf: Box<[u8]> = self.as_bytes().into();
1815        unsafe { from_boxed_utf8_unchecked(buf) }
1816    }
1817}
1818
1819#[stable(feature = "rust1", since = "1.0.0")]
1820impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for Box<T, A> {
1821    #[inline]
1822    fn eq(&self, other: &Self) -> bool {
1823        PartialEq::eq(&**self, &**other)
1824    }
1825    #[inline]
1826    fn ne(&self, other: &Self) -> bool {
1827        PartialEq::ne(&**self, &**other)
1828    }
1829}
1830
1831#[stable(feature = "rust1", since = "1.0.0")]
1832impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for Box<T, A> {
1833    #[inline]
1834    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
1835        PartialOrd::partial_cmp(&**self, &**other)
1836    }
1837    #[inline]
1838    fn lt(&self, other: &Self) -> bool {
1839        PartialOrd::lt(&**self, &**other)
1840    }
1841    #[inline]
1842    fn le(&self, other: &Self) -> bool {
1843        PartialOrd::le(&**self, &**other)
1844    }
1845    #[inline]
1846    fn ge(&self, other: &Self) -> bool {
1847        PartialOrd::ge(&**self, &**other)
1848    }
1849    #[inline]
1850    fn gt(&self, other: &Self) -> bool {
1851        PartialOrd::gt(&**self, &**other)
1852    }
1853}
1854
1855#[stable(feature = "rust1", since = "1.0.0")]
1856impl<T: ?Sized + Ord, A: Allocator> Ord for Box<T, A> {
1857    #[inline]
1858    fn cmp(&self, other: &Self) -> Ordering {
1859        Ord::cmp(&**self, &**other)
1860    }
1861}
1862
1863#[stable(feature = "rust1", since = "1.0.0")]
1864impl<T: ?Sized + Eq, A: Allocator> Eq for Box<T, A> {}
1865
1866#[stable(feature = "rust1", since = "1.0.0")]
1867impl<T: ?Sized + Hash, A: Allocator> Hash for Box<T, A> {
1868    fn hash<H: Hasher>(&self, state: &mut H) {
1869        (**self).hash(state);
1870    }
1871}
1872
1873#[stable(feature = "indirect_hasher_impl", since = "1.22.0")]
1874impl<T: ?Sized + Hasher, A: Allocator> Hasher for Box<T, A> {
1875    fn finish(&self) -> u64 {
1876        (**self).finish()
1877    }
1878    fn write(&mut self, bytes: &[u8]) {
1879        (**self).write(bytes)
1880    }
1881    fn write_u8(&mut self, i: u8) {
1882        (**self).write_u8(i)
1883    }
1884    fn write_u16(&mut self, i: u16) {
1885        (**self).write_u16(i)
1886    }
1887    fn write_u32(&mut self, i: u32) {
1888        (**self).write_u32(i)
1889    }
1890    fn write_u64(&mut self, i: u64) {
1891        (**self).write_u64(i)
1892    }
1893    fn write_u128(&mut self, i: u128) {
1894        (**self).write_u128(i)
1895    }
1896    fn write_usize(&mut self, i: usize) {
1897        (**self).write_usize(i)
1898    }
1899    fn write_i8(&mut self, i: i8) {
1900        (**self).write_i8(i)
1901    }
1902    fn write_i16(&mut self, i: i16) {
1903        (**self).write_i16(i)
1904    }
1905    fn write_i32(&mut self, i: i32) {
1906        (**self).write_i32(i)
1907    }
1908    fn write_i64(&mut self, i: i64) {
1909        (**self).write_i64(i)
1910    }
1911    fn write_i128(&mut self, i: i128) {
1912        (**self).write_i128(i)
1913    }
1914    fn write_isize(&mut self, i: isize) {
1915        (**self).write_isize(i)
1916    }
1917    fn write_length_prefix(&mut self, len: usize) {
1918        (**self).write_length_prefix(len)
1919    }
1920    fn write_str(&mut self, s: &str) {
1921        (**self).write_str(s)
1922    }
1923}
1924
1925#[stable(feature = "rust1", since = "1.0.0")]
1926impl<T: fmt::Display + ?Sized, A: Allocator> fmt::Display for Box<T, A> {
1927    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1928        fmt::Display::fmt(&**self, f)
1929    }
1930}
1931
1932#[stable(feature = "rust1", since = "1.0.0")]
1933impl<T: fmt::Debug + ?Sized, A: Allocator> fmt::Debug for Box<T, A> {
1934    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1935        fmt::Debug::fmt(&**self, f)
1936    }
1937}
1938
1939#[stable(feature = "rust1", since = "1.0.0")]
1940impl<T: ?Sized, A: Allocator> fmt::Pointer for Box<T, A> {
1941    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1942        // It's not possible to extract the inner Uniq directly from the Box,
1943        // instead we cast it to a *const which aliases the Unique
1944        let ptr: *const T = &**self;
1945        fmt::Pointer::fmt(&ptr, f)
1946    }
1947}
1948
1949#[stable(feature = "rust1", since = "1.0.0")]
1950impl<T: ?Sized, A: Allocator> Deref for Box<T, A> {
1951    type Target = T;
1952
1953    fn deref(&self) -> &T {
1954        &**self
1955    }
1956}
1957
1958#[stable(feature = "rust1", since = "1.0.0")]
1959impl<T: ?Sized, A: Allocator> DerefMut for Box<T, A> {
1960    fn deref_mut(&mut self) -> &mut T {
1961        &mut **self
1962    }
1963}
1964
1965#[unstable(feature = "deref_pure_trait", issue = "87121")]
1966unsafe impl<T: ?Sized, A: Allocator> DerefPure for Box<T, A> {}
1967
1968#[unstable(feature = "legacy_receiver_trait", issue = "none")]
1969impl<T: ?Sized, A: Allocator> LegacyReceiver for Box<T, A> {}
1970
1971#[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1972impl<Args: Tuple, F: FnOnce<Args> + ?Sized, A: Allocator> FnOnce<Args> for Box<F, A> {
1973    type Output = <F as FnOnce<Args>>::Output;
1974
1975    extern "rust-call" fn call_once(self, args: Args) -> Self::Output {
1976        <F as FnOnce<Args>>::call_once(*self, args)
1977    }
1978}
1979
1980#[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1981impl<Args: Tuple, F: FnMut<Args> + ?Sized, A: Allocator> FnMut<Args> for Box<F, A> {
1982    extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output {
1983        <F as FnMut<Args>>::call_mut(self, args)
1984    }
1985}
1986
1987#[stable(feature = "boxed_closure_impls", since = "1.35.0")]
1988impl<Args: Tuple, F: Fn<Args> + ?Sized, A: Allocator> Fn<Args> for Box<F, A> {
1989    extern "rust-call" fn call(&self, args: Args) -> Self::Output {
1990        <F as Fn<Args>>::call(self, args)
1991    }
1992}
1993
1994#[stable(feature = "async_closure", since = "1.85.0")]
1995impl<Args: Tuple, F: AsyncFnOnce<Args> + ?Sized, A: Allocator> AsyncFnOnce<Args> for Box<F, A> {
1996    type Output = F::Output;
1997    type CallOnceFuture = F::CallOnceFuture;
1998
1999    extern "rust-call" fn async_call_once(self, args: Args) -> Self::CallOnceFuture {
2000        F::async_call_once(*self, args)
2001    }
2002}
2003
2004#[stable(feature = "async_closure", since = "1.85.0")]
2005impl<Args: Tuple, F: AsyncFnMut<Args> + ?Sized, A: Allocator> AsyncFnMut<Args> for Box<F, A> {
2006    type CallRefFuture<'a>
2007        = F::CallRefFuture<'a>
2008    where
2009        Self: 'a;
2010
2011    extern "rust-call" fn async_call_mut(&mut self, args: Args) -> Self::CallRefFuture<'_> {
2012        F::async_call_mut(self, args)
2013    }
2014}
2015
2016#[stable(feature = "async_closure", since = "1.85.0")]
2017impl<Args: Tuple, F: AsyncFn<Args> + ?Sized, A: Allocator> AsyncFn<Args> for Box<F, A> {
2018    extern "rust-call" fn async_call(&self, args: Args) -> Self::CallRefFuture<'_> {
2019        F::async_call(self, args)
2020    }
2021}
2022
2023#[unstable(feature = "coerce_unsized", issue = "18598")]
2024impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Box<U, A>> for Box<T, A> {}
2025
2026#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
2027unsafe impl<T: ?Sized, A: Allocator> PinCoerceUnsized for Box<T, A> {}
2028
2029// It is quite crucial that we only allow the `Global` allocator here.
2030// Handling arbitrary custom allocators (which can affect the `Box` layout heavily!)
2031// would need a lot of codegen and interpreter adjustments.
2032#[unstable(feature = "dispatch_from_dyn", issue = "none")]
2033impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T, Global> {}
2034
2035#[stable(feature = "box_borrow", since = "1.1.0")]
2036impl<T: ?Sized, A: Allocator> Borrow<T> for Box<T, A> {
2037    fn borrow(&self) -> &T {
2038        &**self
2039    }
2040}
2041
2042#[stable(feature = "box_borrow", since = "1.1.0")]
2043impl<T: ?Sized, A: Allocator> BorrowMut<T> for Box<T, A> {
2044    fn borrow_mut(&mut self) -> &mut T {
2045        &mut **self
2046    }
2047}
2048
2049#[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2050impl<T: ?Sized, A: Allocator> AsRef<T> for Box<T, A> {
2051    fn as_ref(&self) -> &T {
2052        &**self
2053    }
2054}
2055
2056#[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2057impl<T: ?Sized, A: Allocator> AsMut<T> for Box<T, A> {
2058    fn as_mut(&mut self) -> &mut T {
2059        &mut **self
2060    }
2061}
2062
2063/* Nota bene
2064 *
2065 *  We could have chosen not to add this impl, and instead have written a
2066 *  function of Pin<Box<T>> to Pin<T>. Such a function would not be sound,
2067 *  because Box<T> implements Unpin even when T does not, as a result of
2068 *  this impl.
2069 *
2070 *  We chose this API instead of the alternative for a few reasons:
2071 *      - Logically, it is helpful to understand pinning in regard to the
2072 *        memory region being pointed to. For this reason none of the
2073 *        standard library pointer types support projecting through a pin
2074 *        (Box<T> is the only pointer type in std for which this would be
2075 *        safe.)
2076 *      - It is in practice very useful to have Box<T> be unconditionally
2077 *        Unpin because of trait objects, for which the structural auto
2078 *        trait functionality does not apply (e.g., Box<dyn Foo> would
2079 *        otherwise not be Unpin).
2080 *
2081 *  Another type with the same semantics as Box but only a conditional
2082 *  implementation of `Unpin` (where `T: Unpin`) would be valid/safe, and
2083 *  could have a method to project a Pin<T> from it.
2084 */
2085#[stable(feature = "pin", since = "1.33.0")]
2086impl<T: ?Sized, A: Allocator> Unpin for Box<T, A> {}
2087
2088#[unstable(feature = "coroutine_trait", issue = "43122")]
2089impl<G: ?Sized + Coroutine<R> + Unpin, R, A: Allocator> Coroutine<R> for Box<G, A> {
2090    type Yield = G::Yield;
2091    type Return = G::Return;
2092
2093    fn resume(mut self: Pin<&mut Self>, arg: R) -> CoroutineState<Self::Yield, Self::Return> {
2094        G::resume(Pin::new(&mut *self), arg)
2095    }
2096}
2097
2098#[unstable(feature = "coroutine_trait", issue = "43122")]
2099impl<G: ?Sized + Coroutine<R>, R, A: Allocator> Coroutine<R> for Pin<Box<G, A>>
2100where
2101    A: 'static,
2102{
2103    type Yield = G::Yield;
2104    type Return = G::Return;
2105
2106    fn resume(mut self: Pin<&mut Self>, arg: R) -> CoroutineState<Self::Yield, Self::Return> {
2107        G::resume((*self).as_mut(), arg)
2108    }
2109}
2110
2111#[stable(feature = "futures_api", since = "1.36.0")]
2112impl<F: ?Sized + Future + Unpin, A: Allocator> Future for Box<F, A> {
2113    type Output = F::Output;
2114
2115    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
2116        F::poll(Pin::new(&mut *self), cx)
2117    }
2118}
2119
2120#[stable(feature = "box_error", since = "1.8.0")]
2121impl<E: Error> Error for Box<E> {
2122    #[allow(deprecated, deprecated_in_future)]
2123    fn description(&self) -> &str {
2124        Error::description(&**self)
2125    }
2126
2127    #[allow(deprecated)]
2128    fn cause(&self) -> Option<&dyn Error> {
2129        Error::cause(&**self)
2130    }
2131
2132    fn source(&self) -> Option<&(dyn Error + 'static)> {
2133        Error::source(&**self)
2134    }
2135
2136    fn provide<'b>(&'b self, request: &mut error::Request<'b>) {
2137        Error::provide(&**self, request);
2138    }
2139}
2140
2141#[unstable(feature = "pointer_like_trait", issue = "none")]
2142impl<T> PointerLike for Box<T> {}